The present invention relates to an automatic bus transfer device for single phase and multiphase power systems. The automatic bus transfer device provides infinite isolation and automatic switching from a primary power bus to a secondary power bus when the power (or voltage) being provided by the primary power bus falls below a predetermined level.
Both tactical and commercial power supply systems often require uninterrupted power at a desired voltage level or at desired voltage levels. This can be provided using an expensive uninterruptable power system (UPS). UPS arrangements, however, degrade system reliability because an additional single thread of power conditioning is required (e.g., an inverter operated by a battery which is floated on the input of the inverter). If the UPS fails, the entire system is rendered inoperative. An UPS also requires a substantial amount of energy and exhibits a burden relative to the overall operating efficiency of the power supply arrangement. In battery back-up UPS arrangements, the batteries add significant weight to the overall arrangement. A battery also has a limited lifetime and therefore eventually must be replaced.
There are several known techniques and devices for switching between a primary power source and secondary power source. Examples of such techniques and devices can be found aboard tactical platforms, such as between the starboard and port power supplies on military vessels and aircraft, as well as in commercial applications such as hospitals, financial institutions, broadcasting stations, and other facilities where the supply of power is critical.
The majority of critical power supplies use dual sources of power which are arranged so that when one source fails the alternate source is utilized by switching the power load to the alternate source.
A first such arrangement uses magnetic isolation between two sources of power. The requisite source isolation, however, is provided at the expense of considerable weight and volume, as well as structure-borne and airborne noise. Isolation magnetics operating at 1 kVA or more, for example, tend to create severe levels of structure-borne and airborne noise. To overcome the problem of noise, a significant amount of acoustic shielding is required. This, in turn, adds to the overall weight of the power supply system. In addition, the volume of the power supply system is increased by the line frequency isolation magnetics. Decoupling diodes are required at the output from each power factor correction circuit of the power supply system, along with a suitable failure monitoring arrangement. If a transient becomes coupled to either or both power supply buses, the magnetic isolators may become damaged, thereby permanently degrading performance of the power supply arrangement.
FIG. 4 schematically illustrates an example of the first type of dual power supply. The illustrated dual power supply includes two separate EMI filters 402 and two separate power factor correction modules 404. Between each EMI filter 402 and the associated power factor correction module 404, there is an isolation transformer 406.
Each isolation transformer 406 operates at a power level of 130% of the user's required output power. The dual power supply, therefore, suffers from the problems described above. For example, a typical 1 Kw transformer 406 weighs approximately thirty one pounds. The transformers 406 alone therefore account for sixty two pounds in the total weight of a one kilowatt dual power supply.
The arrangement illustrated in FIG. 4 also requires decoupling diodes 408 and a fault reporting arrangement capable of indicating when the decoupling diodes 408 have failed.
As illustrated in FIG. 5, a second type of dual power supply arrangement utilizes input diode bridge rectifiers 500 to provide the requisite isolation between two power supply sources. In particular, two power factor correction circuits 502 (PFC circuits) and two EMI (electromagnetic interference) filters 504 are included in the dual power supply arrangement. Each combination of a PFC circuit 502 and an EMI filter 504 is associated with its respective input diode bridge rectifier 500. The break-down voltage of each input diode bridge rectifier 500 is rated for the imposed hourly maximum test voltage to ensure safe operation (hi-pot peak) of, for example, 1039 volts. A voltage hold-up device 506 is located at the output from the PFC circuit to maintain a desired voltage level, even during transients up to 22 milliseconds in duration.
Typically, such diode bridge rectifier-based arrangements require output PFC decoupling diodes 508 and a fault reporting arrangement capable of indicating when the decoupling diodes have failed. The PFC BITE (power factor correction built-in test equipment) and availability of both buses are reported via a relay or optical isolator. Bus-to-output isolation is provided only by the output modules. Failure of a diode associated with the input diode bridge rectifier may create a safety hazard to personnel as the outputs from both power sources are then connected to one another as a result of this failure.
Further, since each power supply device requires its own EMI filter 504 and its own PFC circuit 502, an additional disadvantage of the input diode bridge rectifier-based arrangement is the need for twice the number of EMI filters and PFC circuits.